In recent years, heating of the earth because of the so-called greenhouse effect due to an increase of atmospheric CO2 has been predicted. In view of this, there is an increased demand for a means of power generation capable of providing clean energy without causing CO2 buildup. Among various proposals which are expected to meet such societal demand, solar cells providing photoelectric conversion are expected to be a future power generation source since they supply electric power notably without consumption of fossil combustible and without emission of CO2 and/or of other pollutants.
Solar cell modules generally comprise at least one photovoltaic element encapsulated between a front layer on its light receiving surface side and a back layer. Front layer, often glazed with glass, is generally provided for assuring weather resistance, anti-scratch and impact resistance, heat resistance, but still enabling access to maximum photoelectric conversion efficiency thanks to transparency in the whole visible spectrum.
On the other side, backside of the module is often covered with a backing layer, generally composed of polymer films and laminates.
The backing layer is generally intended to provide weatherability, physical protection, electrical insulation and moisture protection of the wiring and other sensitive components of the module, while adhering to the other parts of the module. This back layer is typically required to be opaque, so as to provide suitable protection against solar radiation of possibly sensitive layers/components located behind it. Also, in order to minimize losses of light radiation power, it may be required for this back layer to possess reflecting capabilities. Finally, adequate mechanical properties are required.
Among solutions proposed up to now as back-layer, composites comprising a fluoropolymer layer, in particular vinylidene fluoride (VDF) polymer or vinylfluoride (PVF) polymer layers, possibly coupled with other polymer layers (e.g. polyethylene terephthalate) have been proposed in the past.
Nevertheless, fluoropolymer composition providing films fulfilling above mentioned features are still a shortfall in the art.
Actually, fluoropolymer compositions, in particular VDF polymer compositions, comprising inorganic pigments, in particular Zn-based white pigments, are known in the art. Compositions comprising VDF and ZnO or ZnS are notably disclosed in GB 1049089 (PENNSALT CHEMICALS CORP) Nov. 23, 1966, U.S. Pat. No. 4,314,004 (PPG INDUSTRIES INC) Feb. 2, 1982, U.S. Pat. No. 3,839,253 (BALM PAINTS LTD) Jan. 10, 1974.
As mentioned, said compositions have already found application as reflecting/opaque layers in light/optical-related applications, including photovoltaic modules.
Thus, U.S. 2006001978 (ETERNAL CHEMICAL CO LTD) May 1, 2006 discloses a UV-resistant film useful in LCDs as reflective film for back light source, made from a substrate film layer coated with a fluororesin (e.g. a VDF, VF or chlorotrifluoroethylene (CTFE) polymer) admixed with inorganic particles (e.g. ZnO particles) and a fluorescent agent, e.g. ZnS.
Also, WO 2007/079246 (DU PONT (US)) Dec. 7, 2007 and WO 2007/079247 (DU PONT (US)) Dec. 7, 2007 disclose a fluoropolymer coated film useful as backsheet in a photovoltaic module, said film comprising:
a polymer film, preferably a polyester film (providing electrical insulation and moisture barrier properties);
a fluoropolymer coating layer, preferably based on VDF or VF polymers, further comprising pigments or fillers (e.g. metallic oxides; sulfides) having high thermal stability and/or barriers particles (able to reduce permeation to water, solvents and gases), like notably mica particles coated with e.g. ZnO.
Nevertheless, while maximization of inorganic fillers content, in particular of Zn-based fillers content, in the fluororesin composition is recommended for achieving required opacity, whiteness and reflecting properties, processability of composition thereof for manufacturing films is generally heavily affected, so that films of adequate mechanical properties are actually not available.